1. Field of the Invention
This invention relates to a gas sensing element to detect alcoholic gas and those gases giving off an offensive odor such as, for example, methyl mercaptan (CH.sub.3 SH), hydrogen sulfide (H.sub.2 S), and so forth.
2. Description of Prior Art
Concentration measurement of methyl mercaptan, hydrogen sulfide, and other gases which are the principal components of offensive odor emitted from sewage disposing sites, refuse disposing sites, etc. has been done partly in an electro-chemical process. For this purpose, however, sampling of such gases is necessary and procedures for the measurement are also complicated. On the other hand, the conventional semiconductor gas sensor is still not satisfactory in its sensing capability and has not yet found its way into practical use.
As an element to detect inflammable gases, i.e., a gas sensing element, there have been known various types to be mentioned as follows: (a) an element which utilizes variations in electrical conductivity of a material constituting the same due to chemical adsorption of the gases; (b) an element which detects a rise in its temperature due to burning of the inflammable gas which has come into contact with the element; (c) an element which utilizes a gas concentration dependent property of an electromotive force of a solid electrolyte; (d) an element which utilizes a gas concentration dependent property of infrared ray absorption intensity; and others. Of these various types of gas sensing element, the type (a) of the element has been mainly employed for the purpose of continuous measurements or controls of the gases owing to its convenience in use, stability in its operation, heat-resistant property, and so forth. More specifically, this type of gas sensing element utilizes variations in the electrical conductivity of a metal oxide ceramic semi-conductor due to its chemical adsorption of a gas. For the metal oxide to be the base material for the sensing element, there have been mostly used SnO.sub.2, ZnO, etc. as the principal component. While these metal oxides are sensitive to inflammable gases such as methane, propane, hydrogen, carbon monoxide, alcohol, and so on, they have disadvantageously poor selectivity to alcohol. As a matter of fact, it has happened not infrequently that a gas leakage sensor (or a gas alarm) has given off a warning signal even by alcohol vapor from wine while it is being warmed for `hot wine`.